Rule of 72: How Long Until Your Money Doubles

Short answer: the rule of 72 says that money growing at a fixed yearly rate doubles in about 72 ÷ the rate years. At 8% that is 9 years; at 4%, 18 years. The rule is close to exact between about 6% and 10%, a little too slow at low rates and a little too fast at high ones. It works in reverse for debt and inflation: a credit card at the November 2026 average rate of 22.36% doubles an unpaid balance in about 3 years, and 3.7% inflation halves what your cash buys in about 19 years.

Who this is for: savers and investors who want a quick way to compare rates, fees and debt without a spreadsheet. What it does not cover: how to pick investments, and the tax on your returns, which slows doubling in a taxable account. The rule assumes a steady rate and reinvested growth; real markets do neither, and the section on limits says what that means in practice. This is general information, not investment advice.

The ruleHow the rule of 72 works

Divide 72 by the yearly interest rate or return, written as a whole number (8 for 8%, not 0.08). The answer is the number of years it takes for the money to double, if the rate stays the same and the growth stays invested.

It also runs backwards. If you want your money to double in 10 years, you need about 72 ÷ 10 = 7.2% a year. The exact answer is 7.18%.

The same arithmetic applies to anything that grows by a percentage: a debt that is not being paid down, prices under inflation, or a fee that takes a percentage every year. For anything with regular deposits or more detail, use a compound interest calculator, because the rule only handles a single lump sum.

Why 72The math behind the shortcut

Money growing at rate r doubles when (1 + r) raised to the number of years equals 2. Solving for the years gives the exact formula: ln(2) ÷ ln(1 + r). The natural logarithm of 2 is 0.693, so for small rates the answer is close to 69.3 ÷ the rate. That is why 69.3 is exact when interest compounds continuously.

So why 72 and not 69? Two reasons. First, for yearly compounding at normal rates, the right number creeps up as the rate rises: it is about 70 at 2%, 72 at about 7.85%, and 74 at 15%. Second, 72 divides evenly by 1, 2, 3, 4, 6, 8, 9 and 12, so you can do it in your head.

The errors are small where most people use the rule. At 1% the rule says 72 years against an exact 69.7, an error of 2.3 years on a 70-year wait. At 25% it says 2.9 years against 3.1. You will see the rule of 70 (or 69) used for low rates, such as inflation or economic growth, and the rule of 72 for investment returns. The rule of 114 estimates tripling, because ln(3) is about 1.10 and the same upward adjustment applies: at 8%, 114 ÷ 8 = 14.3 years, which is also the exact answer. The rule of 144 is just two doublings: 144 ÷ 8 = 18 years to quadruple.

Compounding more often shortens the wait slightly. At 6% compounded monthly, money doubles in 11.6 years instead of 11.9. Savings accounts quote an APY (annual percentage yield), which already includes compounding, so you can use the APY directly.

In November 2026How long your money takes to double now

Here is the rule applied to the rates on offer this month. Savings and CD rates are the FDIC's national averages, published on October 19, 2026. Treasury yields are from the Treasury's daily yield curve on November 6. The credit card rate is the Federal Reserve's latest figure for accounts that pay interest.

The gap between the average savings account and a high-yield one is the gap between never and once in your working life. Over 10 years, $10,000 at 0.38% grows to $10,387. At 3.20% it grows to $13,702. Our guide to high-yield savings after the Fed's September hike shows what to check before you move cash.

Two cautions on this table. Savings rates move with the Federal Reserve, which held its target range at 3.75%–4.00% on October 28, so the doubling time on a savings account can change next month. And the stock figure is a long-run geometric average: the S&P 500 lost money in many single years, and nobody gets a smooth 10.02%.

Debt doubles tooThe rule on a credit card

The rule does not care whose money it is. The Federal Reserve's G.19 release of November 6 puts the average rate on credit card accounts that pay interest at 22.36% for the third quarter of 2026. Card interest is charged daily, which makes the effective yearly rate 25.05%.

That is why paying down a 22% card beats almost any investment you could make with the same money. Our comparison of paying off debt or investing runs the numbers on lower-rate debt, where the answer is less obvious. If you have several balances, the debt avalanche targets the one that doubles fastest first, and a debt payoff tracker shows each balance shrinking month by month.

InflationHow fast your cash loses half its value

Use the same rule on inflation and it tells you when prices double, which is the same as your cash losing half of what it buys. The Bureau of Labor Statistics measured consumer price inflation at 3.7% for the 12 months to September 2026.

At 3.7%, the rule of 72 gives 19.5 years. The exact answer is 19.1 years. So $100 kept in a drawer would buy about $50 of today's goods by late 2045, if inflation stayed at that rate. At the Federal Reserve's 2% target, the wait is about 35 years.

That does not make a savings account a bad place for an emergency fund. It means cash is for safety and short-term goals, and long-term money needs a return above inflation. Our post on living with 3.7% inflation covers the budget side, and the Inflation Calculator converts any future amount into today's dollars.

FeesWhat a 1% expense ratio does to doubling time

A fee is a negative interest rate that you pay every year, whether the fund goes up or down. Subtract it from the return before you apply the rule.

The 1% fee costs $45,442, or 28.5% of the final balance, even though it looks small each year. Our guide to expense ratios explains where fund fees hide and how to compare them. The same logic applies to an advisory fee charged on top of fund fees: add them together before you subtract.

Worked examplesThree questions the rule answers in seconds

1. How much could $10,000 grow by retirement? Say you are 30 and invest $10,000 in a broad stock index fund until 66, 36 years. At the long-run S&P 500 average of 10.02%, the rule says a doubling every 7.2 years, so five doublings: $10,000 becomes about $320,000. The exact figure is $310,982, because 10% is above the rule's sweet spot. Real results will differ, possibly by a lot, because returns are not steady. If you are deciding whether to invest a lump sum at once or in parts, see lump sum vs dollar-cost averaging.

2. What rate do I need to double my money in 10 years? 72 ÷ 10 = 7.2%. The exact rate is 7.18%. That is above any Treasury yield in November 2026, so a 10-year doubling needs at least part of the money in stocks or other riskier assets.

3. Is a 4% CD that locks my money for 5 years worth it? At 4% your money doubles in about 18 years, so 5 years gets you less than a third of a doubling. The rule helps you see that the choice between 3.5% and 4% matters far less than the choice between 0.38% and either of them.

For retirement savings, every year counts more than any rate tweak: one extra doubling at the end adds as much as the whole balance had reached before it. That is why maxing out a 401(k) early matters, and why the 4% rule assumes your money keeps compounding during retirement. A portfolio tracker shows your actual return each year, which is the number to plug into the rule.

HistoryWhere the rule comes from, and the Einstein myth

The earliest known written version is in Luca Pacioli's Summa de arithmetica, printed in Venice in 1494. Pacioli, the Italian mathematician also known for describing double-entry bookkeeping, gives the rule without explaining it. That suggests merchants were already using it, since logarithms were not invented until more than a century later.

You will often see the claim that Albert Einstein called compound interest "the eighth wonder of the world" or "the most powerful force in the universe". There is no reliable evidence that he said either. Quote Investigator found no significant evidence for the attribution, and found similar praise of compound interest in print as early as 1916. Snopes rates the claim as undetermined. The sentiment is fair; the attribution is not.

LimitsWhere the rule of 72 misleads

Where these numbers come fromSources and method

Every doubling time in this article is calculated in a small script (drafts/rule-of-72-numbers.mjs) using the exact formula ln(2) ÷ ln(1 + r) and checked with asserts. Rates were read on November 9, 2026 from: the FDIC's national rates page (savings 0.38%, 12-month CD 1.76%, dated October 19, 2026); Ally's rate page (3.20% APY, as of November 5); Wealthfront's rate page (3.55% base APY, as of October 1); the U.S. Treasury's daily par yield curve for November 6 (3-month 4.19%, 10-year 4.86%); the Federal Reserve's G.19 consumer credit release of November 6 (22.36% on accounts assessed interest, third quarter 2026); and the BLS consumer price index for September 2026 (3.7%).

The S&P 500 figure is our calculation from Professor Aswath Damodaran's NYU Stern historical returns data: $100 invested at the start of 1928 grew to $1,157,598.95 by the end of 2025 with dividends reinvested, a geometric average of 10.02% over 98 years. It is before taxes, fees and inflation, so it flatters the result most people actually get.

Treasury yields are treated as yearly rates. They are quoted on a semiannual basis, which would shorten those two doubling times by about two months. The credit card example ignores payments and fees.

Common questionsFAQ

Is the rule of 72 accurate?
Within a few months for rates between about 4% and 15%. It is almost exact near 8%. At very low rates it overstates the time slightly, and at very high rates it understates it.

Should I use 69, 70 or 72?
Use 72 for investment returns between about 6% and 10%. Use 70 for low rates such as inflation or GDP growth. Use 69.3 for continuous compounding. The differences are small next to the uncertainty in any future return.

Does the rule of 72 work with monthly contributions?
No. It applies to a single sum left to grow. For regular deposits, use a compound interest calculator.

How long does it take to double money at 7%?
About 72 ÷ 7 = 10.3 years. The exact answer with yearly compounding is 10.2 years.

Can I use the rule for debt?
Yes. A balance at 22.36% that is not being paid down doubles in about 3 years. That is the strongest argument for paying off high-rate debt first.

Who invented the rule of 72?
Nobody knows. The earliest written version is in Luca Pacioli's Summa de arithmetica of 1494. Einstein did not invent it, and there is no reliable evidence he praised compound interest in the words often quoted.

The takeawayDivide by 72, then compare

Before you choose a savings account, a fund or which debt to pay first, divide 72 by each rate. The answers in years make the trade-offs plain: 183 years for an average savings account, 22 for a good one, about 7 for the long-run stock market, and 3 for a credit card working against you. Then subtract fees and inflation, and run the real numbers in a compound interest calculator.